1. Field of the Invention
This invention relates to the production of regularly varying amounts of electric power, and more particularly, to the conversion of carbonaceous materials such as coal, into both gaseous and liquid fuels which can be effectively utilized so as to generate variable amounts of electric power for both industrial and utility needs. This invention also relates to the capture, storage and release of carbon monoxide by forming and dissociating a suitable organic molecule, i.e., an alkyl formate, to exploit the natural stoichiometry of the coal conversion to greatest advantage.
2. Discussion of the Prior Art
Electric power production and distribution grids can generally be characterized as needing to respond to power demand patterns which vary over time. Such demand patterns generally can cyclically rise and fall over daily, weekly and even annual periods, with the precise degree of variation being substantially different in various localities. It is not uncommon for the base load and peak load facilities of a utility grid to use different technologies and or fuels.
As is well known to those in the art, current conventional electric power generation plants frequently utilize fuel oil or natural gas as the sources of energy for the generation of electrical power. However, these fuels, which are particularly attractive for supplying increased electric power during peak demand periods, are no longer as inexpensive and in such plentiful supply as they have been in the past. Now, due to the high cost of crude petroleum, refined petroleum products and natural gas, as well as the unreliability of the sources of these fuels, it has become necessary that different energy sources be explored and new techniques for the effective utilization of both old and other sources of energy be developed. Coal, which is in great abundance and is relatively inexpensive, is a natural material for the art to investigate as a primary energy source for the generation of electric power.
Originally the primary source of heat to generate electric and mechanical power, coal had fallen out of favor due both to the problems involved in handling, transport and storage, as well as its content of ash, sulfur and other impurities which can create environmental and other emissions control problems. But now, because of its lower cost and more secure domestic supply, coal is returning to favor, and more efficient and cleaner means of utilization are under investigation.
Coal is usually combusted with air, with the heat of reaction produced used to generate a high pressure steam which is expanded in a turbine to generate mechanical or electrical energy. The electric industry has developed a variety of large, highly efficient generators which can be driven by expanding steam. Coal fired steam generators, however, are not well suited for producing greatly varying amounts of electricity, but rather are usually designed for more of a base (substantially constant) load. Coal combustors are also poorly suited to interrupted requirements. Usually they are preferred for base load operations because of the lower fuel cost. Recently, to provide for peak load and reserve load demands, open-cycle gas turbines using liquid or gaseous hydrocarbon fuels have often been utilized due to their quick startup capability and relatively low capital cost.
Gas turbines presently require a clean burning fuel which is non-corrosive to the turbine blades, and unfortunately some carbonaceous fuels are corrosive. Similar problems have also arisen from the burning of soot and ash producing liquid petroleum products.
Coal and other solid carbonaceous materials, as mentioned above, further contain a substantial amount of sulfur compounds, the combustion of which creates serious environmental problems. Since enormous volumes of low pressure gas are produced in the combustion of these sulfurbearing coals, it is very expensive to remove the polluting sulfur compounds such as SO.sub.2 and SO.sub.3 following combustion. These and other problems have thus spurred the search for coal gasification processes which will produce a clean fuel gas in which the sulfur compounds have been removed from the fuel prior to combustion. Coal can be gasified with the resulting gasification products (syngas) cleaned and used to power gas turbines, which are easily implemented to the production of electric power. However, the gasification processes usually produce a syngas possessing a much lower heating value than clean natural gas. With its high heating value, natural gas is also economical for long range distribution and the present pipeline system provides a reservoir for meeting demand variations. The alternative solution of producing large on-site gasification and associated power generation plants has not appeared to be economical for producing widely variable amounts of fuel needed for peak load generation of power, since it is too expensive to store such gaseous products and too large a capital expense to provide for such greatly increased gas production rate for peak load demand.
One attempt the art has proposed to satisfy the requirements of clean power production coupled with variable power demands is disclosed in Report AP-2212, "Economic Evaluation of the Co-production of Methanol and Electricity with Texaco Gasification Combined-Cycle Systems", of the Electric Power Research Institute (EPRI) Palo Alto, Calif., wherein a process of gasifying coal at a constant rate under high pressure and temperature is set forth. The process recovers waste heat for internal usage and cleans the raw syngas of sulfur compounds and other contaminants, followed by feeding the cleaned syngas to a so-called partial-conversion, "once-through" (no gas recycle) methanol synthesis reaction, with the unconverted syngas burned for direct base load power generation, thereby replacing more expensive, equivalently cleaned fuels. The synthesized methanol is stored and later used as fuel for gas turbine systems during the peak demand periods. Unfortunately, a once-through process is limited by both stoichiometry and process efficiency in the proportion of storable, clean fuel which can be extracted from the syngas. The gasification process produces a synthesis gas having, typically, a 1.2/1 ratio of CO to H.sub.2, together with lesser amounts of CO.sub.2, H.sub.2 S, methane and other inerts. Since the synthesis of methanol consumes two moles of H.sub.2 per mole of CO it is readily apparent that even if H.sub.2 conversion is complete, this stoichiometric requirement will limit the conversion of the syngas stream. Since only a limited fraction, typically about 50% of the available hydrogen is converted in the once-through methanol synthesis, the process will convert a maximum of only about 25% of the available syngas to a storable liquid methanol fuel. In the conventional production of methanol, the CO component of the syngas is partially shifted by reaction with water vapor in the well known water-gas reaction to produce CO.sub.2 and additional H.sub.2, to thereby arrive at the appropriate 1/2 ratio of CO/H.sub.2 for the essentially complete conversion of the gas to methanol. This results in the loss of thermodynamic efficiency and also requires more capital equipment. This process uses a low conversion per pass reaction which requires a compressive recycle to react large volumes of unconverted gas. In the EPRI Report a once-through methanol synthesis is shown, when operated as part of a coal gasification combined cycle plant, to be a more economical means for a utility to obtain a supply of clean methanol for peak load power turbine fuel than by purchasing methanol produced from coal, or the like.
U.S. Pat. No. 3,868,817 discloses a process for the generation of mechanical and electrical power from a purified fuel gas produced from solid carbonaceous fuels. The purified fuel gas is used to generate power using gas turbines.
U.S. Pat. Nos. 3,986,349 and 4,092,825 disclose a process for generating electrical power from solid carbonaceous materials in open-cycle gas turbines to meet variable power demands. The process involves the conversion of coal to a combustible synthesis gas formed by its reaction with steam and oxygen. The synthesis gas is then divided into two portions, one of which is contacted with both Fischer-Tropsch and hydrogenation catalysts to produce a variety of synthetic hydrocarbons ranging from methane and ethane to C.sub.22 or higher. The normally gaseous portion of the product is separated and recombined with the second portion of the synthesis gas stream, and the combined streams are subsequently combusted and utilized as fuels in an electricity-generating gas turbine. The normally liquid hydrocarbon products (C.sub.5 -C.sub.22) are stored and also utilized as fuel for gas turbines to produce supplemental power for peak-load demand. This is a complex and torturous scheme, with substantial thermodynamic losses. Most of the reacted syngas is converted to a variety of products, only some of which are liquids. Water is also formed from CO and H.sub.2 along with some oxygen-free hydrocarbons, resulting in a loss of potential fuel to the power system. Such a process is therefore unattractive for variable power production applications.
U.S. Pat. No. 4,341,069 disclosed a process for satisfying variable electrical power generation requirements through the use of coal to produce syngas and dimethyl ether, which are both fired in turbinecompressor arrangements driving electrical generators, wherein the storable ether fuel is used to supplement the base power load produced by the firing and expanding syngas in the appropriate gas turbine generator units. This route is also limited in flexibility by the large proportion of hydrogen to carbon monoxide in dimethyl ether, in cases of syngases produced from coal or similar carbonaceous feeds to gasification, which are more abundant in carbon monoxide than hydrogen.
U.S. Pat. No. 3,716,619 discloses the recovery of carbon monoxide from fuel gases for immediate release as a chemical synthesis feedstock. The process does not separate the homogeneous sodium methoxide catalyst from the alkyl formate solution produced, and thus the mixed reactor effluent would not be suitable for storage, since the CO would be spontaneously released under practical bulk storage conditions, in an undesired time period.
Processes disclosing the production of synthesis gas from coal which are useful in electric power generation plants are set forth in U.S. Pat. Nos. 4,094,143, 4,227,416 and 4,132,065.
The troublesome problems presented by widely variable power load demand, combined with the special fuel requirements necessary for clean, efficient and non-corrosive power generation units, are substantially avoided by the process of the invention, which combines the production of a base load power generating capability with the additional production from CO and lower alkyl alcohols such as methanol of an energy storing, simple molecular compound, i.e., alkyl formates, such as methyl formate, which are particularly useful in the efficient and economic generation of the necessary peak load power requirements, through the release of CO for use during periods of increased demand, as well as the regeneration of the alkyl alcohol, thereby permitting a continuous cycle of energy capture for release during peak demand periods.